1. Field of the Invention
In the present invention, electric signals having phases differences different from one another are applied to a complex resonator having natural resonance, thereby making it possible to continuously sweep a resonance frequency of the resonator.
The present invention is composed of a resonator made up of discrete electron devices or a resonator including a piezoelectric resonator, a Micro Electro Mechanical System (MEMS) resonator, an optical resonator and the like. The present invention relates to frequency variable devices such as ultrasonic transducers, radio antennas, photoelectric converters, and frequency oscillators.
2. Description of the Related Art
A frequency source (time source) with high signal purity is required for communication or observation equipment. Furthermore, there is a demand that the frequency of this frequency source should be variable over a broad frequency range. The required frequency range covers all the frequency domains from the ultrasonic range to the electromagnetic wave range, the lightwave range and the like.
Patent Documents 1 and 2 disclose an ultrasonic transducer for the ultrasonic range, the frequency of which, however, cannot be varied continuously.
Patent Document 3 discloses an antenna which can be switched between frequencies. However, the frequencies cannot be varied continuously.
Patent Document 4 discloses a method of varying a resonance frequency in the electromagnetic wave range by changing a signal path length by switches. However, the manner of varying the resonance frequency is gradual but not continuous, and thus, not suited for cases where precise frequency changes are required.
Patent Document 5 discloses a laser diode having three electrodes to output a lightwave whose wavelength is variable. However, the diode cannot produce a high-purity wavelength output because the dimension of the resonator is changed in this method.
Patent Documents 6 and 7 disclose a method of continuously varying a frequency of a ultrasonic wave by combining a piezoelectric resonator and a phase shifter.
However, a variable range of the frequency cannot exceed limits of a frequency range determined by an electromechanical coupling factor peculiar to the piezoelectric phenomenon. Further, this method has a critical defect that noise power within the range increases and thus purity of the frequency deteriorates as the variable range is made broader.
Patent Document 8 discloses a technique of controlling directivity of an antenna by changing the phase. However, it is impossible to vary a frequency at maximum sensitivity of the antenna.
Patent Document 9 discloses a method of continuously varying an antiresonance frequency of ultrasonic or electromagnetic wave by changing power distribution applied to two resonator units. However, since this method utilizes an antiresonance frequency, which has very small power value, this method has a defect that the frequency purity degrades due to external noises, and needs further improvements.
Patent Document 1:                Japanese Patent Application Kokai No. 11-295281 (Japanese Patent Application No. 10-114130)        
Patent Document 2:                Japanese Patent Application Kokai No. 10-174436 (Japanese Patent Application No. 8-346591)        
Patent Document 3:                Japanese Patent Application Kohyo No. 2004-519150 (Japanese Patent Application No. 2002-563613)        
Patent Document 4:                Japanese Patent Application Kokai No. 2005-253059 (Japanese Patent Application No. 2005-26019)        
Patent Document 5:                Japanese Patent Application Kokai No. 2004-223111 (Japanese Patent Application No. 2004-28852)        
Patent Document 6:                Japanese Patent Application Kokai No. 2000-196358 (Japanese Patent Application No. 10-377053)        
Patent Document 7:                Japanese Patent Application Kokai No. 10-313216 (Japanese Patent Application No. 9-122493)        
Patent Document 8:                Japanese Patent Application Kokai No. 2006-109425 (Japanese Patent Application No. 2005-257600)        
Patent Document 9:                International Publication Pamphlet WO2006/046672        